Photodetector and method for manufacturing photodetector

ABSTRACT

A photodetector includes: a substrate; an optical fiber disposed on the substrate; and a photodetection element fixed to the substrate, and that detects scattered light of light guided by the optical fiber. The photodetector further includes: a first fixing member and a second fixing member that fix the optical fiber to the substrate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a U.S. national stage application of International ApplicationNo. PCT/JP2018/002931 filed on Jan. 30, 2018, which claims priority fromJapanese Patent Application Nos. 2017-017430, 2017-017476, and2017-017477, all three of which were filed on Feb. 2, 2017. The contentsof these applications are incorporated herein in their entirety.

TECHNICAL FIELD

The present invention relates to a photodetector and a method formanufacturing a photodetector.

BACKGROUND

In the related art, a photodetector as disclosed in Patent Document 1has been known. The photodetector includes a photodetection element thatdetects scattered light of light guided by an optical fiber.

In this type of photodetector, since the detection result of thescattered light changes depending on the relative positions of thephotodetection element and the optical fiber, it is necessary to fix thephotodetection element and the optical fiber by a fixing member or thelike.

[Patent Document 1] Japanese Patent Publication No. 2013-508688

Meanwhile, when the temperature of the fixing member changes due to achange in ambient temperature or the like, the fixing member may expandor contract, which may cause a shift in the relative positions of thephotodetection element and the optical fiber. As a result, the detectionresult of the scattered light changes with the temperature change of thefixing member.

SUMMARY

One or more embodiments of the present invention provide a photodetectorcapable of limiting relative positional deviation between aphotodetection element and an optical fiber caused by a temperaturechange.

In one or more embodiments, a photodetector includes a substrate, anoptical fiber placed on the substrate; and a photodetection element thatis fixed to the substrate, and is configured to detect scattered lightof light guided by the optical fiber.

A photodetector according to one or more embodiments may further includea first fixing member and a second fixing member that fix the opticalfiber to the substrate, in which the first fixing member is disposed onan opposite side of the photodetection element from the second fixingmember in a longitudinal direction in which the optical fiber extends,when a volume of a portion of the first fixing member on a first sideacross the optical fiber in a transverse direction orthogonal to thelongitudinal direction is V₁ and a volume of a portion of the firstfixing member on a second side opposite to the first side is V₂, in topview, and a volume of a portion of the second fixing member on the firstside of the optical fiber in the transverse direction is V₃, and avolume of a portion of the second fixing member on the second side isV₄, in top view, either V₁>V₂ and V₃<V₄, or V₁<V₂ and V₃>V₄ issatisfied.

According to one or more embodiments, since the first fixing member andthe second fixing member are configured to satisfy any one of V₁>V₂ andV₃<V₄ or V₁<V₂ and V₃>V₄, the optical fiber is rotationally movedrelative to the photodetection element as the temperature changes. Thus,for example, as compared with the case where the photodetector isconfigured to satisfy V₁>V₂ and V₃>V₄, and the optical fiber moves inparallel with the photodetection element, it is possible to reduce therelative positional deviation between the optical fiber and thephotodetection element due to the temperature change.

In a photodetector according to one or more embodiments, when a linearexpansion coefficient of a material forming the first fixing member isα₁, and a linear expansion coefficient of a material forming the secondfixing member is α₂, and in top view, a distance between a center ofgravity of the first fixing member and the optical fiber in thetransverse direction is X₁, and a distance between a center of gravityof the second fixing member and the optical fiber in the transversedirection is X₂, α₁/α₂=X₂/X₁ is satisfied.

According to one or more embodiments, since the optical fiber rotatesaround the vicinity of the photodetection element due to a temperaturechange, it is possible to more reliably reduce the relative positionaldeviation between the photodetection element and the optical fiber.

A method for manufacturing a photodetector according to one or moreembodiments is a method for manufacturing a photodetector including asubstrate, an optical fiber which is placed on the substrate, aphotodetection element which is fixed to the substrate, and isconfigured to detect scattered light of light guided by the opticalfiber, and a first fixing member and a second fixing member which fixthe optical fiber to the substrate, the first fixing member beingdisposed on an opposite side of the photodetection element from thesecond fixing member in a longitudinal direction in which the opticalfiber extends, the method including: a first application step ofapplying a resin to be the first fixing member to the substrate and theoptical fiber; a volume detection step of detecting a volume V₁ of aportion of the first fixing member on a first side across the opticalfiber in a transverse direction orthogonal to the longitudinaldirection, and a volume V₂ of a portion of the first fixing member on asecond side opposite to the first side, in top view; and a secondapplication step of applying a resin to be the second fixing member tothe substrate and the optical fiber, based on a detection result in thevolume detection step, in which when, in top view, a volume of a portionof the second fixing member on the first side in the transversedirection is V₃, and a volume of a portion of the second fixing memberon the second side is V₄, a discharge amount of the resin to be thesecond fixing member is controlled such that either V₁>V₂ and V₃<V₄ orV₁<V₂ and V₃>V₄ is satisfied, in the second application step.

According to the manufacturing method of one or more embodiments, sincethe resin to be the second fixing member is discharged based on thedetection results of the volumes V₁, V₂ of the first fixing member, forexample, in a case where the first fixing member is applied unevenly tothe optical fiber in the transverse direction, the second fixing memberis formed by controlling a discharge amount of the resin to be thesecond fixing member such that the relative positional deviation betweenthe photodetection element and the optical fiber due to the temperaturechange.

A photodetector according to one or more embodiments of the presentinvention further includes a first fixing member and a second fixingmember which fix the optical fiber to the substrate, in which the firstfixing member is disposed on an opposite side of the photodetectionelement from the second fixing member in a longitudinal direction inwhich the optical fiber extends, and the first fixing member is formedof a material having a positive linear expansion coefficient, and thesecond fixing member is formed of a material having a negative linearexpansion coefficient.

According to the photodetector of one or more embodiments, the opticalfiber is fixed to the substrate by the first fixing member and thesecond fixing member. Further, the first fixing member is formed of amaterial having a positive linear expansion coefficient, and the secondfixing member is formed of a material having a negative linear expansioncoefficient. Therefore, in a case where a temperature change occurs inthe first fixing member and the second fixing member, one of the firstfixing member and the second fixing member contracts, and the otherexpands. The first fixing member and the second fixing member aredisposed on both sides of the photodetection element in the longitudinaldirection of the optical fiber. Thus, for example, in a case where boththe first fixing member and the second fixing member are formed of amaterial having a positive linear expansion coefficient, it is possibleto limit the relative positional deviation between the optical fiber andthe photodetection element caused by expansion of both the first fixingmember and the second fixing member. Further, for example, in a casewhere both the first fixing member and the second fixing member areformed of a material having a negative linear expansion coefficient, itis possible to limit the application of tension to the optical fiberbecause both first fixing member and the second fixing member contract.

In a photodetector according to one or more embodiments of the presentinvention, the second fixing member is formed of a material having anegative linear expansion coefficient in the longitudinal direction.

In a photodetector according to one or more embodiments of the presentinvention, an absolute value of the linear expansion coefficient of thematerial forming the second fixing member is larger than an absolutevalue of the linear expansion coefficient of the material forming thefirst fixing member.

According to one or more embodiments, since the contraction amount ofthe second fixing member exceeds the expansion amount of the firstfixing member when the temperature of the photodetector rises, it ispossible to more reliably limit bending of the optical fiber, forexample, in the vertical direction or the horizontal direction betweenthe first fixing member and the second fixing member, and a change inthe position with respect to the photodetection element.

In a photodetector according to one or more embodiments of the presentinvention, when the linear expansion coefficient of a material formingthe first fixing member is α_(A), and the absolute value of a linearexpansion coefficient of a material forming the second fixing member isα_(B), and the distance between the center of gravity of the firstfixing member and the optical fiber is X_(0A), and the distance betweenthe center of gravity of the second fixing member and the optical fiberis X_(0B), the value of α_(A)/α_(B) and the value of X_(0B)/X_(0A) areapproximately the same.

According to one or more embodiments, in a case where the optical fiberis disposed out of the ideal design position, the optical fiber moves soas to rotate around the vicinity of the photodetection element as thetemperature changes. Therefore, the amount of change in the distance ofthe optical fiber to the photodetection element depending on thetemperature can be further reduced. Thereby, the temperature dependencyof the detection result by the photodetection element can be furtherreduced.

A photodetector according to one or more embodiments further includes afirst fixing member and a second fixing member which fix the opticalfiber to the substrate; and a fixing base which fixes the photodetectionelement to the substrate, in which the fixing base has at least oneopening, a portion of the optical fiber is accommodated inside thefixing base through the opening, and the at least one opening is closedby either the first fixing member or the second fixing member.

According to one or more embodiments, the opening for introducing theoptical fiber to the inside of the fixing base is closed by the fixingmember. Therefore, it is possible to prevent dust or the like fromentering the fixing base through an opening and affecting the detectionresult of the scattered light by the photodetection element.

In a photodetector according to one or more embodiments, a part ofeither the first fixing member or the second fixing member is locatedinside the fixing base.

According to one or more embodiments, the effect of fixing the opticalfiber to the substrate by the first fixing member or the second fixingmember can be further enhanced. Further, it is possible to prevent dustor the like from entering a space where the light receiving surface ofthe photodetection element and the optical fiber face each other. Then,the detection result of the scattered light by the photodetectionelement can be further stabilized.

In a photodetector according to one or more embodiments, the fixing basehas a main body that holds the photodetection element, and a width ofeither the first fixing member or the second fixing member is largerthan a width of the main body.

According to one or more embodiments, the contact area of the firstfixing member or the second fixing member and the substrate isincreased. Therefore, the connection strength between the first fixingmember or the second fixing member and the substrate increases, and theoptical fiber can be fixed to the substrate more securely.

In a photodetector according to one or more embodiments, the fixing basehas a main body that holds the photodetection element, and a width ofeither the first fixing member or the second fixing member is smallerthan a width of the main body.

According to one or more embodiments, the area of the portion of thesubstrate covered by the first fixing member or the second fixing memberis reduced. Therefore, since the area of the substrate on which othercomponents can be mounted increases, the mounting density of componentscan be raised.

A photodetector according to one or more embodiments further includes: aconnection member which is fixed to the substrate in contact with amounting surface of the substrate, has a placing surface on which theoptical fiber is placed, and connects the optical fiber located on theplacing surface and the substrate; a photodetection element that detectsscattered light of light guided by the optical fiber; and a fixing basewhich fixes the photodetection element to the substrate, and expands andcontracts at least in a vertical direction with a temperature change, inwhich the fixing base has at least one opening, and a contact surfacefixed in contact with the mounting surface, a portion of the opticalfiber is accommodated inside the fixing base through the opening, theplacing surface is positioned at least a portion facing thephotodetection element across at least the optical fiber, and theconnection member expands and contracts at least in the verticaldirection with a temperature change such that the distance in thevertical direction between the optical fiber located on the placingsurface and the photodetection element is within a predetermined range.

According to the photodetector in one or more embodiments, thephotodetection element is fixed to the substrate through the fixingbase, and is fixed in a state where the contact surface of the fixingbase is in contact with the mounting surface of the substrate.Therefore, when the temperature of the photodetector rises or falls, thefixing base thermally expands or thermally contracts. Therefore, thephotodetection element moves in the vertical direction with respect tothe mounting surface of the substrate.

On the other hand, the optical fiber is placed on the placing surface ofthe connection member, and the connection member is fixed in contactwith the mounting surface of the substrate and connects the opticalfiber located on the placing surface and the substrate. Further, theplacing surface is disposed at a portion facing at least thephotodetection element with at least the optical fiber interposedtherebetween. Then, the connection member expands and contracts at leastin the vertical direction with the temperature change such that thedistance in the vertical direction between the optical fiber placed onthe placing surface and the photodetection element is within apredetermined range. Thus, as compared with, for example, a case wherethe optical fiber is directly placed on the mounting surface of thesubstrate, it is possible to limit the relative positional deviationbetween the photodetection element and the optical fiber in the verticaldirection caused by the temperature change.

In a photodetector according to one or more embodiments, the fixing basehas a positioning unit which determines a position of the photodetectionelement in the vertical direction with respect to the mounting surface,a linear expansion coefficient of a material forming the connectionmember is α_(a), a linear expansion coefficient of a material formingthe fixing base is α_(b), a thickness in the vertical direction of aportion of the connection member on which the optical fiber is placed isH_(a0), and a length in the vertical direction from the contact surfaceto the positioning portion is H_(b0),α_(a)×H_(a0)(α_(a)×H_(a0)−2×α_(b)×H_(b0))<0 is satisfied.

According to one or more embodiments, the connection member and thefixing base are configured to satisfyα_(a)×H_(a0)(α_(a)×H_(a0)−2×α_(b)×H_(b0))<0. Thus, since the thermalexpansion amount or thermal contraction amount of the connection memberexcessively exceeds the thermal expansion amount or thermal contractionamount of the fixing base, and the connection member is provided, it ispossible to limit an increase in relative positional deviation in thevertical direction caused by the temperature change of thephotodetection element and optical fiber, and to more reliably achievethe above-described effects of the photodetector.

In a photodetector according to one or more embodiments, a value ofα_(b)×H_(b0) and a value of α_(a)×H_(a0) are substantially the same.

According to one or more embodiments, the amount of movement of thephotodetection element in the vertical direction with respect to themounting surface of the substrate due to thermal expansion or thermalcontraction of the fixing base and the amount of movement of the opticalfiber in the vertical direction with respect to the mounting surface ofthe substrate due to thermal expansion or thermal contraction of theconnection member are substantially the same. Thus, even if atemperature change occurs, the photodetection element and the opticalfiber are displaced while maintaining the relative positionalrelationship in the vertical direction, and it is possible to morereliably limit the relative positional deviation in the verticaldirection of the both caused by the temperature change.

According to one or more embodiments of the present invention, it ispossible to provide a photodetector capable of limiting relativepositional deviation between a photodetection element and an opticalfiber caused by a temperature change.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram showing a configuration of a laser systemprovided with a photodetector according to one or more embodiments.

FIG. 2 is a perspective view of the photodetector of one or moreembodiments.

FIG. 3 is a top view of the photodetector of one or more embodiments.

FIG. 4 is a cross-sectional view of the photodetector taken along lineA-A in FIG. 3.

FIG. 5 is a cross-sectional view of the photodetector taken along lineB-B in FIG. 3.

FIG. 6A is a top view showing a photodetector of a comparative examplein which a first fixing member and a second fixing member are formed tobe biased to the −X side, and shows a state before a temperature change.

FIG. 6B is a view showing the state of FIG. 6A after the temperaturechange.

FIG. 7A is a top view of the photodetector of one or more embodiments,showing a state before a temperature change.

FIG. 7B is a view showing a state of FIG. 7A after the temperaturechange.

FIG. 8 is a top view of the photodetector of one or more embodiments.

FIG. 9 is a block diagram showing a configuration of a laser systemprovided with a photodetector of one or more embodiments.

FIG. 10 is a perspective view of the photodetector of one or moreembodiments.

FIG. 11 is a top view of the photodetector of one or more embodiments.

FIG. 12 is a cross-sectional view of the photodetector taken along lineA-A in FIG. 11.

FIG. 13 is a cross-sectional view of the photodetector taken along lineB-B in FIG. 11.

FIG. 14A is an explanatory diagram of a case where a temperature of thephotodetector of the comparative example rises.

FIG. 14B is an explanatory diagram of a case where a temperature of thephotodetector of one or more embodiments rises.

FIG. 15A is a top view showing a state in which the position of theoptical fiber is shifted in the photodetector shown in FIG. 11, andshows a state before a temperature change.

FIG. 15B is a view showing a state of FIG. 15A after the temperaturechange.

FIG. 16 is a block diagram showing a configuration of a laser systemprovided with a photodetector of one or more embodiments.

FIG. 17 is a perspective view of the photodetector of one or moreembodiments.

FIG. 18 is a top view of the photodetector of one or more embodiments.

FIG. 19 is a cross-sectional view of the photodetector taken along lineA-A in FIG. 18.

FIG. 20 is a cross-sectional view of the photodetector taken along lineB-B in FIG. 18.

FIG. 21 is a graph showing a fluctuation of a detection value by thephotodetection element according to the temperature according to one ormore embodiments.

DETAILED DESCRIPTION

The configuration of a photodetector according to one or moreembodiments will be described below with reference to FIGS. 1 to 8

In order to facilitate understanding of the invention, in FIGS. 1 to 8,the scales of components are appropriately changed.

FIG. 1 is a block diagram showing the configuration of a laser systemprovided with a photodetector 1A of one or more embodiments.

As shown in FIG. 1, a laser system LS includes a plurality of laserdevices 31, a combiner 32 (multiplexer), an optical fiber F10 (outputoptical fiber), a photodetector 1A, and a control device 33 (controlunit). The laser system LS outputs output light L11 (laser light) fromthe output end X of the optical fiber F10.

The laser device 31 is a device that outputs a laser beam under thecontrol of the control device 33.

The combiner 32 optically combines the plurality of beams of outputlight L1 output from the plurality of laser devices 31. Inside thecombiner 32, the optical fibers F extending from respective laserdevices 31 are bundled into one (made into one by melt drawing), and theone optical fiber is fusion-spliced to one end of the optical fiber F10.The optical fiber F10 is an optical fiber functioning as a transmissionmedium, and guides the output light L11 (light obtained by opticallycombining a plurality of beams of output light L1 output from the laserdevices 31 by the combiner 32). The output light L11 guided by theoptical fiber F10 is output from the output end X of the optical fiberF10.

The control device 33 controls the plurality of laser devices 31 suchthat the power of the output light L11 output from the output end Xbecomes constant, based on the detection result to be described later ofthe photodetector 1A to be described later.

The photodetector 1A is disposed between the combiner 32 and the outputend X, and is configured to detect the power of light guided by theoptical fiber F10. The photodetector 1A may be disposed between thelaser device 31 and the combiner 32, and may detect the power of lightguided by the optical fiber F.

FIG. 2 is a perspective view of the photodetector 1A. As shown in FIG.2, the photodetector 1A includes a substrate 2, an optical fiber F10 oran optical fiber F (hereinafter simply referred to as an optical fiberF10) placed on the substrate 2, a first fixing member 3, a second fixingmember 4, a fixing base 5, and a photodetection element 6.

Here, in one or more embodiments, an XYZ orthogonal coordinate system isset, and the positional relationship of each configuration will bedescribed. A Y direction is the extending direction (longitudinaldirection) of the optical fiber F10 in a state before the optical fiberF10 moves due to a temperature change. A Z direction is a direction(vertical direction) perpendicular to the surface of the substrate 2 onwhich the optical fiber F10 is placed. In the Z direction, the side ofthe substrate 2 on which the optical fiber F10 is placed is referred toas the upper side, and the opposite side is referred to as the lowerside. A X direction (transverse direction) is a direction orthogonal toboth the Y direction and the Z direction.

Further, in top view of the substrate (plan view), a first side of theoptical fiber F10 in the X direction may be referred to as the −X side,and a second side may be referred to as the +X side.

FIG. 3 is a top view of the photodetector 1A. FIG. 4 is across-sectional view taken along line A-A in FIG. 3, and the outline ofthe second fixing member 4 is indicated by a two-dot chain line. FIG. 5is a cross-sectional view taken along line B-B in FIG. 3.

As shown in FIG. 2 and the like, the fixing base 5 has a main body 51that holds the photodetection element 6 and a fastening portion 52 forfastening the fixing base 5 to the substrate 2. The fastening portion 52of the fixing base 5 is fixed to the substrate 2 by a screw 8. Thefixing base 5 is formed in a substantially rectangular parallelepipedshape having a depth of 20 mm, a width of 20 mm, and a height of 8 mm.As a material of the fixing base 5, for example, aluminumsurface-treated with matte black alumite can be used. As shown in FIGS.4 and 5, a through hole 5 a and a groove 5 b are formed in the fixingbase 5. The through hole 5 a penetrates the main body 51 of the fixingbase 5 in the Z direction, and extends perpendicularly to the substrate2. The groove 5 b is formed on the bottom surface of the main body 51 ofthe fixing base 5 and extends over the entire length of the fixing base5 in the X direction. As shown in FIGS. 4 and 5, the width in the Xdirection and the height in the Z direction of the groove 5 b are largerthan the diameter of the optical fiber F10.

As shown in FIG. 5, the fixing base 5 has openings 5 b 1, 5 b 2 of thegroove 5 b. A part of the optical fiber F10 is introduced into thegroove 5 b through the openings 5 b 1, 5 b 2 and is accommodated insidethe fixing base 5. The first opening 5 b 1 is closed by the first fixingmember 3, and the second opening 5 b 2 is closed by the second fixingmember 4. A part of the first fixing member 3 enters the groove 5 bthrough the first opening 5 b 1 and is located inside the fixing base 5.A part of the second fixing member 4 enters the groove 5 b through thesecond opening 5 b 2 and is located inside the fixing base 5.

As shown in FIG. 5, the photodetection element 6 is formed with acylindrical portion 6 a and a flange portion 6 b. The cylindricalportion 6 a extends in the Z direction, and the flange portion 6 bextends in a plane orthogonal to the Z direction. When the cylindricalportion 6 a is fitted in the through hole 5 a of the fixing base 5, thepositions of the photodetection element 6 in the X and Y directions withrespect to the fixing base 5 are determined. Further, in a state wherethe lower surface of the flange portion 6 b is in contact with the uppersurface of the fixing base 5, the photodetection element 6 is fixed tothe fixing base 5 by the screw 7. Thereby, the position of thephotodetection element 6 in the Z direction with respect to the fixingbase 5 is determined.

With the above configuration, the photodetection element 6 is fixed tothe fixing base 5 in a state where the positions in the X direction, theY direction, and the Z direction with respect to the fixing base 5 aredetermined. Further, since the fixing base 5 is fixed to the substrate 2by the screws 8, the photodetection element 6 is fixed to the substrate2 through the fixing base 5. That is, the fixing base 5 fixes thephotodetection element 6 to the substrate 2. Thus, the distance Lbetween the lower end surface (hereinafter referred to as the lightreceiving surface 6 c) of the cylindrical portion 6 a of thephotodetection element 6 and the outer peripheral surface of the opticalfiber F10 is determined.

The photodetection element 6 receives the scattered light (for example,Rayleigh scattered light) from the optical fiber F10 at the lightreceiving surface 6 c, and converts the intensity of the scattered lightinto electric power. The electric power is amplified on an electriccircuit board (not shown) and input to the control device 33. Thus, thecontrol device 33 can monitor the power of the light guided by theoptical fiber F10 in real time. For example, a PIN photodiode can beused as the photodetection element 6. In a case where a PIN photodiodeis used as the photodetection element 6, the distance L from the outerperipheral surface of the optical fiber F10 to the light receivingsurface 6 c is about several millimeters.

The first fixing member 3 and the second fixing member 4 fix the opticalfiber F10 to the substrate 2. The first fixing member 3 and the secondfixing member 4 are disposed on both sides of the photodetection element6 in the X direction. As shown in FIGS. 2 to 5, the first fixing member3 and the second fixing member 4 are formed in a substantiallyquarter-sphere shape. Parts of the first fixing member 3 and the secondfixing member 4 respectively enter the groove 5 b of the fixing base 5.

As shown in FIG. 4, the width of the first fixing member 3 or the secondfixing member 4 in the X direction is referred to as a width W1. Thewidth of the main body 51 in the X direction is referred to as a widthW2. As shown in FIGS. 2 and 4, an end face of the main body 51, havingthe first opening 5 b 1 or the second opening 5 b 2, is referred to asan end face 51 a. The end face 51 a faces in the Y direction. As shownin FIG. 4, the width W1 of the first fixing member 3 or the secondfixing member 4 in the X direction (direction substantially orthogonalto the surface direction of the end face 51 a) when viewed from the Ydirection (longitudinal direction of the optical fiber F10) Is largerthan the width W2 of the main body 51 in the X direction.

The width of the first fixing member 3 or the second fixing member 4 inthe Y direction (the surface direction of the end face 51 a) in the mainbody 51 is referred to as a width W3. In one or more embodiments, thewidth W1 and the width W3 of the first fixing member 3 or the secondfixing member 4 are larger than the width W2 of the main body 51. Thus,the contact area of the first fixing member 3 and the substrate 2 isincreased, and the contact area of the second fixing member 4 and thesubstrate 2 is increased. Therefore, the connection strength between thefirst fixing member 3 and the substrate 2 increases, and the connectionstrength between the second fixing member 4 and the substrate 2increases, and the optical fiber F10 can be fixed to the substrate 2more securely.

The first fixing member 3 and the second fixing member 4 are each formedof a material having a positive linear expansion coefficient. As amaterial of these fixing members, for example, a silicon resin having alinear expansion coefficient of about 300×10⁻⁶ [/K] can be used. Thespecific materials of the first fixing member 3 and the second fixingmember 4 may be the same as or different from each other.

As shown in FIGS. 2 to 5, the space where the light receiving surface 6c of the photodetection element 6 and the optical fiber F10 face eachother is sealed by the photodetection element 6, the fixing base 5, thefirst fixing member 3, and the second fixing member 4. Morespecifically, the first fixing member 3 and the second fixing member 4close the openings 5 b 1, 5 b 2 of the groove 5 b formed in the fixingbase 5.

With this configuration, it is possible to prevent dust or the like fromentering the space where the light receiving surface 6 c of thephotodetection element 6 and the optical fiber F10 face each other andaffecting the detection result of the scattered light by thephotodetection element 6.

Further, in one or more embodiments, a part of either the first fixingmember 3 or the second fixing member 4 is located inside the fixing base5. With this configuration, the effect of fixing the optical fiber F10to the substrate 2 can be further enhanced, compared with the case wherea part of either the first fixing member 3 or the second fixing member 4is not located inside the fixing base 5. Further, it is possible to morereliably prevent dust or the like from entering the fixing base 5. Then,the detection result of the scattered light by the photodetectionelement 6 can be further stabilized. In addition, a part of the firstfixing member 3 or the second fixing member 4 is located inside thefixing base 5.

Meanwhile, FIG. 3 shows the case where the first fixing member 3 and thesecond fixing member 4 are formed in such a shape that the temperaturedependency of the detection result of the photodetection element 6becomes small. Specifically, in top view, the volume V₁ of the portionof the first fixing member 3 on the first side (−X side) of the opticalfiber F10 in the X direction and the volume V₂ of the portion on thesecond side (+X side) are equal to each other. Similarly, in top view,the volume V₃ of the portion of the second fixing member 4 on the firstside (−X side) of the optical fiber F10 in the X direction and thevolume V₄ of the portion on the second side (+X side) are equal to eachother. That is, since the first fixing member 3 and the second fixingmember 4 are formed such that V₁=V₂ and, V₃=V₄, the temperaturedependency of the photodetection element 6 is small. At this time, asshown in FIG. 3, the center of gravity C3 of the first fixing member 3and the center of gravity C4 of the second fixing member 4 are locatedon the optical fiber F10. A straight line connecting the centers ofgravity C3, C4 in this ideal state is referred to as a center line O.

Here, since the first fixing member 3 and the second fixing member 4 areformed by curing the molten resin, for example, the first fixing member3 and the second fixing member 4 are formed to be biased to the sameside in the transverse direction.

For example, as shown in FIG. 6A, the case is considered where the firstfixing member 3 and the second fixing member 4 are formed at positionsshifted to the −X side with respect to the optical fiber F10. In thiscase, V₁>V₂ and V₃>V₄, and the centers of gravity C3, C4 of therespective fixing members are shifted to the −X side.

FIG. 6B shows a state in which the temperature rises from the state ofFIG. 6A and the first fixing member 3 and the second fixing member 4thermally expand. The solid lines in FIG. 6B indicate the positions andshapes of the first fixing member 3, the second fixing member 4 and theoptical fiber F10 after thermal expansion, and the broken lines indicatethem before thermal expansion.

As shown in FIG. 6B, with the thermal expansion of the first fixingmember 3 and the second fixing member 4, the optical fiber F10 moves tothe +X side. This is because the first fixing member 3 and the secondfixing member 4 expand around the centers of gravity C3, C4. Since themovement amount AX of the optical fiber F10 due to thermal expansion isproportional to temperature, the distance between optical fiber F10 andphotodetection element 6 changes with temperature. Thus, the detectionresult of the photodetection element 6 differs according to thetemperature of the photodetector 1A, resulting in temperaturedependency.

Therefore, in order to reduce the temperature dependency, thephotodetector 1A of one or more embodiments is formed with the firstfixing member 3 and the second fixing member 4 so as to satisfy eitherV₁>V₂ and V₃≤V₄ or V₁<V₂ and V₃>V₄. Hereinafter, a method ofmanufacturing the photodetector 1A will be described with reference toFIGS. 7A and 7B.

When manufacturing the photodetector 1A, first, the optical fiber F10 isplaced on the upper surface of the substrate 2. Next, the fixing base 5is fixed to the substrate 2 with the screw 8, in a state where theoptical fiber F10 is arranged along the groove 5 b of the fixing base 5.Next, the photodetection element 6 is fixed to the fixing base 5 by thescrew 7.

Next, the heated and melted resin to be the first fixing member 3 isdischarged from the tip of the nozzle N1 shown in FIG. 7A and appliedonto the substrate 2 and the optical fiber F10. The application amountof the resin (first resin) to be the first fixing member 3 is, forexample, about 0.5 ml. The applied resin is cooled and solidified toform the first fixing member 3 (first application step).

Here, the tip of the nozzle N1 is disposed immediately above the opticalfiber F10. The example of FIG. 7A shows a state in which the firstfixing member 3 is formed to be biased to the −X side, and V₁>V₂. Atthis time, the center of gravity C3 of the first fixing member 3 isshifted to the −X side.

Next, the volume V₁ of the portion on the −X side of the optical fiberF10 and the volume V₂ of the portion on the +X side of the formed firstfixing member 3 are detected (volume detection step). The volumes V₁, V₂can be detected, for example, by an image recognition device (notshown). In addition, in the volume detection step, the volumes V₁, V₂may be detected before the first fixing member 3 is cured, or thevolumes V₁, V₂ may be detected after the first fixing member 3 is cured.

Next, the resin (second resin) to be the second fixing member 4 isrespectively discharged from the plurality of nozzles N2, N3 which areseparately disposed on both the +X side and the −X side of the opticalfiber F10, and is applied onto the substrate 2 and the optical fiber F10(second application step). The total amount of resin to be the secondfixing member 4 discharged from the nozzles N2, N3 is, for example,about 0.5 ml. The tip portions (discharge holes) of the nozzles N2, N3are respectively disposed at equal intervals on the +X side and the −Xside from the optical fiber F10. The resin discharged from the nozzlesN2, N3 merges in the vicinity of the optical fiber F10 and is cooled toform the second fixing member 4.

Here, the discharge amount of at least one of the nozzles N2, N3 iscontrolled based on the detection result in the volume detection step.Specifically, for example, in a case where the volume detection resultis V₁>V₂, it is controlled such that the discharge amount from thenozzle N2 is larger than the discharge amount from the nozzle N3. Thus,as shown in FIG. 7A, the second fixing member 4 is formed to be biasedto the +X side, and V₃<V₄. At this time, the center of gravity C4 of thesecond fixing member 4 is shifted to the +X side.

In addition, in a case where the volume detection result is V₁<V₂, thedischarge amounts from the nozzles N2, N3 are controlled such thatV₃>V₄. Further, in a case where the volume detection result is V₁=V₂,the discharge amounts from the nozzles N2, N3 are controlled such thatV₃=V₄.

Next, the operation of the photodetector 1A manufactured in this mannerwill be described.

FIG. 7B shows a state in which the temperature of the photodetector 1Arises from the state of FIG. 7A and the first fixing member 3 and thesecond fixing member 4 thermally expand. As shown in FIG. 7B, since thefirst fixing member 3 expands around the center of gravity C3, theoptical fiber F10 is moved toward the +X side. On the other hand, sincethe second fixing member 4 expands around the center of gravity C4, theoptical fiber F10 is moved toward the −X side.

Thus, since the optical fiber F10 is rotated about the point P in thevicinity of the photodetection element 6, for example, as compared tothe case where V₁>V₂ and V₃>V₄ as shown in FIG. 6A, the movement amountof the optical fiber F10 relative to the photodetection element 6 can bereduced.

In addition, the above-described operation is achieved even in the casewhere the photodetector 1A is configured to have V₁<V₂ and V₃>V₄.

Next, as shown in FIG. 8, in a case where V₁>V₂ and V₃<V₄, therelationship between the optical fiber F10 with respect to the centersof gravity C3, C4 and the linear expansion coefficients of the firstfixing member 3 and the second fixing member 4 is considered.

As described above, the first fixing member 3 and the second fixingmember 4 expand or contract around their respective centers of gravityC3, C4 with temperature change. In one or more embodiments, in order toreduce the temperature dependency of the photodetection element 6, theoptical fiber F10 is moved so as to rotate around a point P which is inthe vicinity of the photodetection element 6. By moving in this manner,it is possible to minimize the change in the positional relationshipbetween the optical fiber F10 and the light receiving surface 6 c of thephotodetection element 6.

As shown in FIG. 8, the distance in the X direction between the centerof gravity C3 of the first fixing member 3 and the optical fiber F10 inthe top view is X₁. The distance in the X direction between the centerof gravity C4 of the second fixing member 4 and the optical fiber F10 inthe top view is X₂. Further, the linear expansion coefficient of thefirst fixing member 3 is α₁, and the linear expansion coefficient of thesecond fixing member 4 is α₂.

Here, after there is a temperature change of AT from the state shown inFIG. 8, the distance in the X direction between the center of gravity C3and the optical fiber F10 is X_(1ΔT), and the distance in the Xdirection between the center of gravity C4 and the optical fiber F10 isX_(2ΔT).

At this time, X_(1ΔT) and X_(2ΔT) can be expressed by the followingexpressions (1), (2).

X _(1ΔT) =X ₁×(α₁ ×ΔT+1)  (1)

X _(2ΔT) =X ₂×(α₂ ×ΔT+1)  (2)

The conditions for the optical fiber F10 to rotate around the point Pcan be represented by following expression (3).

X _(1ΔT) =X _(2ΔT)  (3)

By substituting Expressions (1), (2) into both sides of Expression (3)and arranging, the following Expression (4) is obtained.

α₁/α₂ =X ₂ /X ₁  (4)

By setting each condition so as to satisfy the above Expression (4), theoptical fiber F10 rotates around the point P in the vicinity of thephotodetection element 6. Therefore, the temperature dependency of thedetection result of the scattered light can be reduced.

In addition, both the first fixing member 3 and the second fixing member4 may be formed of a material having a positive linear expansioncoefficient, and both may be formed of a material having a negativelinear expansion coefficient. As a material having a negative linearexpansion coefficient, for example, a synthetic resin as described inJapanese Patent No. 5699454 can be used.

In a case where both the first fixing member 3 and the second fixingmember 4 are formed of a material having a negative linear expansioncoefficient, for example, each fixing member expands when thetemperature of the photodetector 1A rises. Therefore, it is possible tolimit the bending of the optical fiber F10.

As described above, in the photodetector 1A of one or more embodiments,the volumes of the first fixing member 3 and the second fixing member 4are configured to satisfy either V₁>V₂ and V₃≤V₄ or V₁<V₂ and V₃>V₄, theoptical fiber F10 is rotationally moved relative to the photodetectionelement 6 with the temperature change. Thus, for example, it isconfigured to satisfy V₁>V₂ and V₃>V₄, as compared with the case wherethe optical fiber F10 moves in parallel with the photodetection element6, it is possible to reduce the relative positional deviation betweenthe optical fiber F10 and the photodetection element 6 due to thetemperature change.

Further, in a case where it is configured to satisfy α₁/α₂=X₂/X₁, theoptical fiber F10 rotates around the vicinity of the photodetectionelement 6, it is possible to more reliably reduce the relativepositional deviation between the photodetection element 6 and theoptical fiber F10 due to a temperature change.

According to the manufacturing method of the photodetector 1A of one ormore embodiments, since the resin to be the second fixing member 4 isdischarged from a plurality of nozzles N2, N3 disposed on both sides ofthe optical fiber F10, based on the detection results of the volumes V₁,V₂ of the first fixing member 3, for example, in a case where the firstfixing member 3 is applied unevenly to the optical fiber F10 in the Xdirection, the second fixing member 4 is formed by controlling adischarge amount from the plurality of nozzles N2, N3 such that therelative positional deviation between the photodetection element 6 andthe optical fiber F10 due to the temperature change.

Further, by controlling the discharge amount from at least one of theplurality of nozzles N2, N3 so as to satisfy either V₁>V₂ and V₃<V₄ orV₁<V₂ and V₃>V₄, the first fixing member 3 and the second fixing member4 are formed such that the optical fiber F10 is rotationally moved witha temperature change, positional deviation between the optical fiber F10and the photodetection element 6 due to temperature change can be morereliably reduced.

It should be noted that the technical scope of the present invention isnot limited to the above-described embodiments, and variousmodifications can be made without departing from the spirit of thepresent invention.

For example, in the embodiments described above, the photodetectionelement 6 is fixed to the substrate 2 using the fixing base 5, but thephotodetection element 6 may be fixed to the substrate 2 by anotherconfiguration without using such a fixing base 5.

Although FIG. 7 shows an example in which the discharge holes of thenozzles N2, N3 are disposed at equal intervals in the transversedirection from the optical fiber F10, the present invention is notlimited to this. For example, the positions of the nozzles N2,N3 may bemoved so as to satisfy either V₁>V₂ and V₃<V₄ or V₁<V₂ and V₃>V₄, bychanging the positions of the discharge holes of the nozzles N2,N3.

In addition, FIG. 7 shows an example in which the resin to be the secondfixing member 4 is discharged from the two nozzles N2, N3, but thepresent invention is not limited to this. For example, by dischargingthe resin to be the second fixing member 4 by one nozzle whose dischargehole is positioned on the center line O, either V₁>V₂ and V₃<V₄ or V₁<V₂and V₃>V₄ may be satisfied. Alternatively, by discharging the resin tobe the second fixing member 4 from any one of the nozzle N2 and thenozzle N3, either V₁>V₂ and V₃<V₄ or V₁<V₂ and V₃>V₄ may be satisfied.

Further, in one or more embodiments, the width W1 and the width W3 arelarger than the width W2, but this relationship may be different. Thatis, the width W1 or the width W3 may be smaller than the width W2. Inthis case, the area of the portion of the substrate 2 covered by thefirst fixing member 3 or the second fixing member 4 is reduced.Therefore, since the mounting area of the other components on thesubstrate 2 is increased, it is possible to realize the photodetector 1Ain which the mounting density of components is improved.

Further, the main body 51 of the fixing base 5 may be formed in a plateshape having a very small width in the Y direction. In this case, thefixing base 5 may not have the groove 5 b, and may be provided with onlythe openings 5 b 1, 5 b 2.

The configuration of a photodetector 1B according to the one or moreembodiments will be described below with reference to FIGS. 9 to 13.

In order to facilitate understanding of the invention, in FIGS. 9 to 13,the scales of components are appropriately changed.

FIG. 9 is a block diagram showing the configuration of a laser system LSprovided with a photodetector 1B of one or more embodiments.

As shown in FIG. 9, the laser system LS includes a plurality of laserdevices 31, a combiner 32 (multiplexer), an optical fiber F10 (outputoptical fiber), the photodetector 1B, and a control device 33 (controlunit). The laser system LS outputs output light L11 (laser light) fromthe output end X of the optical fiber F10.

The laser device 31 is a device that outputs a laser beam under thecontrol of the control device 33.

The combiner 32 optically combines the plurality of beams of outputlight L1 output from the plurality of laser devices 31. Inside thecombiner 32, the optical fibers F extending from respective laserdevices 31 are bundled into one (made into one by melt drawing), and theone optical fiber is fusion-spliced to one end of the optical fiber F10.The optical fiber F10 is an optical fiber functioning as a transmissionmedium, and guides the output light L11 (light obtained by opticallycombining a plurality of beams of output light L1 output from the laserdevices 31 by the combiner 32). The output light L11 guided by theoptical fiber F10 is output from the output end X of the optical fiberF10.

The control device 33 controls the plurality of laser devices 31 suchthat the power of the output light L11 output from the output end Xbecomes constant, based on the detection result to be described later ofthe photodetector 1B to be described later.

The photodetector 1B is disposed between the combiner 32 and the outputend X, and detects the power of light guided by the optical fiber F10.In addition, the photodetector 1B may be disposed between the laserdevice 31 and the combiner 32, and may detect the power of light guidedby the optical fiber F.

FIG. 10 is a perspective view of the photodetector 1B. As shown in FIG.10, the photodetector 1B includes a substrate 2, an optical fiber F10 oran optical fiber F (hereinafter simply referred to as an optical fiberF10) placed on the substrate 2, a first fixing member 3, a second fixingmember 4, a fixing base 5, and a photodetection element 6.

In addition, in one or more embodiments, a direction in which theoptical fiber F10 extends in a state before the optical fiber F10 movesdue to a temperature change is referred to as a longitudinal direction.Further, a direction which is perpendicular to the surface of thesubstrate 2 on which the optical fiber F10 is placed is referred to asthe vertical direction. The vertical direction is orthogonal to thelongitudinal direction. In the vertical direction, the side of thesubstrate 2 on which the optical fiber F10 is placed is referred to asthe upper side, and the opposite side is referred to as the lower side.Further, a direction orthogonal to the longitudinal direction and thevertical direction is referred to as the horizontal direction.

FIG. 11 is a top view of the photodetector 1B. FIG. 12 is across-sectional view taken along line A-A in FIG. 11, and the outline ofthe second fixing member 4 is indicated by a two-dot chain line. FIG. 13is a cross-sectional view taken along line B-B in FIG. 11.

As shown in FIG. 10 and the like, the fixing base 5 is fixed to thesubstrate 2 by a screw 8. The fixing base 5 is formed in a rectangularparallelepiped shape having a depth of 20 mm, a width of 20 mm, and aheight of 8 mm. As a material of the fixing base 5, for example,aluminum surface-treated with matte black alumite can be used. As shownin FIGS. 12 and 13, a through hole 5 a and a groove 5 b are formed inthe fixing base 5. The through hole 5 a penetrates the fixing base 5 inthe vertical direction, and extends perpendicularly to the substrate 2.The groove 5 b is formed on the bottom surface of the fixing base 5 andextends over the entire length of the fixing base 5 in the longitudinaldirection. As shown in FIGS. 12 and 13, the width in the horizontaldirection and the height in the vertical direction of the groove 5 b arelarger than the diameter of the optical fiber F10.

As shown in FIG. 13, the photodetection element 6 is formed with acylindrical portion 6 a and a flange portion 6 b. The cylindricalportion 6 a extends in the vertical direction, and the flange portion 6b extends in a plane orthogonal to the vertical direction. When thecylindrical portion 6 a is fitted in the through hole 5 a of the fixingbase 5, the positions of the photodetection element 6 in thelongitudinal direction and the horizontal direction with respect to thefixing base 5 are determined. Further, in a state where the lowersurface of the flange portion 6 b is in contact with the upper surfaceof the fixing base 5, the photodetection element 6 is fixed to thefixing base 5 by the screw 7. Thus, the position of the photodetectionelement 6 in the vertical direction with respect to the fixing base 5 isdetermined.

With the above configuration, the photodetection element 6 is fixed tothe fixing base 5 in a state where the positions in the longitudinaldirection, the horizontal direction, and the vertical direction withrespect to the fixing base 5 are determined. Further, since the fixingbase 5 is fixed to the substrate 2 by the screws 8, the photodetectionelement 6 is fixed to the substrate 2 through the fixing base 5. Thatis, the fixing base 5 fixes the photodetection element 6 to thesubstrate 2. Thus, the distance L between the lower end surface(hereinafter referred to as the light receiving surface 6 c) of thecylindrical portion 6 a of the photodetection element 6 and the outerperipheral surface of the optical fiber F10 is determined.

The photodetection element 6 receives the scattered light (for example,Rayleigh scattered light) from the optical fiber F10 at the lightreceiving surface 6 c, and converts the intensity of the scattered lightinto electric power. The electric power is amplified on an electriccircuit board (not shown) and input to the control device 33. Thus, thecontrol device 33 can monitor the power of the light guided by theoptical fiber F10 in real time. For example, a PIN photodiode can beused as the photodetection element 6. In a case where a PIN photodiodeis used as the photodetection element 6, the distance L from the outerperipheral surface of the optical fiber F10 to the light receivingsurface 6 c is about several millimeters.

The first fixing member 3 and the second fixing member 4 fix the opticalfiber F10 to the substrate 2. The first fixing member 3 and the secondfixing member 4 are disposed on both sides of the photodetection element6 in the longitudinal direction. As shown in FIGS. 10 to 13, the firstfixing member 3 and the second fixing member 4 are each formed in asubstantially quarter-sphere shape. Parts of the first fixing member 3and the second fixing member 4 respectively enter the groove 5 b of thefixing base 5.

The first fixing member 3 is formed of a material having a positivelinear expansion coefficient. As a material of the first fixing member3, for example, a silicon resin having a linear expansion coefficient ofabout 300×10⁻⁶ [/K] can be used.

The second fixing member 4 is formed of a material having a negativelinear expansion coefficient (for example, the material described inJapanese Patent No. 5699454).

In addition, the second fixing member 4 in one or more embodiments isformed of a material having a negative linear expansion coefficient inthe longitudinal direction. Further, the second fixing member 4 in oneor more embodiments is formed of a material whose absolute value of thelinear expansion coefficient is larger than the absolute value of thelinear expansion coefficient of the material forming the first fixingmember 3.

In addition, the materials of the first fixing member 3 and the secondfixing member 4 described above are only an example, and other materialsmay be used as long as they have a positive linear expansion coefficientand a negative linear expansion coefficient, respectively.

In addition, as shown in FIGS. 10 to 13, the space where the lightreceiving surface 6 c of the photodetection element 6 and the opticalfiber F10 face each other is sealed by the photodetection element 6, thefixing base 5, the first fixing member 3, and the second fixing member4. More specifically, the first fixing member 3 and the second fixingmember 4 close the opening of the groove 5 b formed in the fixing base5.

With this configuration, it is possible to prevent dust or the like fromentering the space where the light receiving surface 6 c of thephotodetection element 6 and the optical fiber F10 face each other andaffecting the detection result of the scattered light by thephotodetection element 6.

When assembling the photodetector 1B, first, the optical fiber F10 isplaced on the upper surface of the substrate 2. Next, the fixing base 5is fixed to the substrate 2 with the screw 8, in a state where theoptical fiber F10 is arranged along the groove 5 b of the fixing base 5.Next, the photodetection element 6 is fixed to the fixing base 5 by thescrew 7. Next, the first fixing member 3 and the second fixing member 4which are heated and melted are applied in the vicinity of both ends ofthe groove 5 b of the fixing base 5 in the longitudinal direction. Theapplication amounts of the first fixing member 3 and the second fixingmember 4 are, for example, about 0.5 ml, respectively. Thus, the firstfixing member 3 and the second fixing member 4 are formed in asubstantially quarter-sphere shape with a radius of about 6 mm, and aportion of the first fixing member 3 and the second fixing member 4enters the groove 5 b. When the first fixing member 3 and the secondfixing member 4 are cooled and solidified, the optical fiber F10 isfixed to the substrate 2 by the first fixing member 3 and the secondfixing member 4.

Next, the operation of the photodetector 1B configured as describedabove will be described in comparison with the photodetector 100 of thecomparative example.

The photodetector 100 of Comparative Example includes a fixing member 40formed of the same material as the first fixing member 3 instead of thesecond fixing member 4 in the photodetector 1B, as shown in FIG. 14A.

FIG. 14A is a view showing a state in which the temperature of thephotodetector 100 of Comparative Example rises, and the shapes of thefirst fixing member 3 and the fixing member 40 before deformation due tothe temperature rise is shown by a two-dot chain line.

Since the first fixing member 3 and the fixing member 40 are formed of amaterial having a positive linear expansion coefficient, it expand asthe temperature rises, as shown in FIG. 14A. Therefore, in the opticalfiber F10, all the portions fixed by the first fixing member 3 and thefixing member 40 move in the longitudinal direction toward thephotodetection element 6. Thus, as shown in FIG. 14A, a part of theoptical fibers F10 facing the photodetection element 6 may bend.Further, the deflection is not limited to the vertical direction, butmay occur in the horizontal direction. In these cases, the distancebetween the optical fiber F10 and the light receiving surface 6 c of thephotodetection element 6 changes. Thus, the detection result of thescattered light by the photodetection element 6 changes.

In addition, in a case where the temperature falls, the first fixingmember 3 and the fixing member 40 both contract. Theremore, thetemperature-dependent tension acts on the portion of the optical fiberF10 facing the photodetection element 6 in the vertical direction. Thetension may affect the detection result of the scattered light by thephotodetection element 6.

As described above, in the photodetector 100 of the comparative example,temperature dependency occurs in the detection result of the scatteredlight by the photodetection element 6.

On the other hand, in the photodetector 1B of one or more embodiments,the above-described temperature dependency can be reduced.

FIG. 14B is a view showing a state in which the temperature of thephotodetector 1B of FIG. 13 rises, and the shapes of the first fixingmember 3 and the second fixing member 4 before deformation due to thetemperature rise is shown by a two-dot chain line.

Since the first fixing member 3 is formed of a material having apositive linear expansion coefficient, it expands as the temperaturerises, as shown in FIG. 14B. On the other hand, since the second fixingmember 4 is formed of a material having a negative linear expansioncoefficient, the second fixing member 4 contracts as the temperaturerises, as shown in FIG. 14B. Thus, even if the portion of the opticalfiber F10 fixed by the first fixing member 3 moves in the longitudinaldirection toward the photodetection element 6 side due to expansion ofthe first fixing member 3, the second fixing member 4 contracts toabsorb the movement in the longitudinal direction, and can prevent theoptical fiber F10 from bending. That is, in a case where the temperaturerises, the portion of the optical fiber F10 facing the photodetectionelement 6 in the vertical direction is moved from the first fixingmember 3 side to the second fixing member 4 side, in the direction ofthe arrow shown in FIG. 14B. Thus, the bending of the optical fiber F10can be limited.

Further, in a case where the temperature falls, the first fixing member3 contracts and the second fixing member 4 expands, the portion of theoptical fiber F10 facing the photodetection element 6 in the verticaldirection is moved from the second fixing member 4 side to the firstfixing member 3 side, in the longitudinal direction. Thus, the bendingof the optical fiber F10 can be limited.

Here, a case is considered where the position of the optical fiber F10is fixed by being shifted from the ideal position in design. FIG. 11shows the case where the optical fiber F10 is at an ideal position indesign. The ideal position in design is a position along a center line Oconnecting the center of gravity (hereinafter referred to as the centerof gravity C3) of the first fixing member 3 and the center of gravity ofthe second fixing member 4 (hereinafter referred to as the center ofgravity C4).

On the other hand, in the state shown in FIG. 15A, the position of theoptical fiber F10 is shifted from the center line O. More specifically,the optical fiber F10 deviates from the center of gravity C3 by X_(0A),and deviates from the center of gravity C4 by X_(0B).

As shown in FIG. 15A, when the optical fiber F10 is fixed at a positionshifted from the center line O, in a case where the temperature rises,the position of the optical fiber F10 changes as shown in FIG. 15B.Specifically, as the first fixing member 3 expands, the portion of theoptical fiber F10 fixed to the first fixing member 3 moves in thehorizontal direction so as to be away from the center of gravity C3.Then, as the second fixing member 4 contracts, the portion of theoptical fiber F10 fixed to the second fixing member 4 moves in theleft-right direction so as to approach the center of gravity C4. Due tothis change in position, the positional relationship between the lightreceiving surface 6 c of the photodetection element 6 and the opticalfiber F10 may change, and a temperature dependency may occur in thedetection result of the scattered light. Therefore, conditions forreducing this temperature dependency are examined.

In order to reduce the temperature dependency as described above, in oneor more embodiments, the optical fiber F10 is moved so as to rotate in aplane orthogonal to the vertical direction around the point P which isin the vicinity of the photodetection element 6. By moving in thehorizontal direction in this manner, it is possible to minimize thechange in the positional relationship between the optical fiber F10 andthe light receiving surface 6 c of the photodetection element 6. Thecondition for rotating the optical fiber F10 in the plane orthogonal tothe vertical direction about the point P is that the movement amount ofthe optical fiber F10 in the horizontal direction due to the temperaturechange is equal in the portion fixed to the first fixing member 3 andthe portion fixed to the second fixing member 4.

As shown in FIG. 15B, the movement amount of the portion of the opticalfiber F10 fixed to the first fixing member 3 in the horizontal directionis X_(ΔTA), and the movement amount of the portion fixed to the secondfixing member 4 in the horizontal direction is X_(ΔTB). The linearexpansion coefficient of the material forming the first fixing member 3is α_(A), and the absolute value of the linear expansion coefficient ofthe material forming the second fixing member 4 is α_(B). Further, theamount of change in temperature from the state shown in FIG. 15A is ΔT.At this time, X_(ΔTA) and X_(ΔTB) can be expressed by the followingexpressions (5), (6).

X _(ΔTA) =X _(0A) ×α _(A) ×ΔT  (5)

X _(ΔTB) =X _(0B)×α_(B) ×ΔT  (6)

The conditions for the optical fiber F10 to rotate in a plane orthogonalto the vertical direction around the point P can be represented byfollowing Expression (7).

X _(ΔTA) =X _(ΔTB)  (7)

By substituting Expressions (5), (6) into both sides of Expression (7)and arranging, the following Expression (8) is obtained.

α_(A)/α_(B) =X _(0B) /X _(0A)  (8)

By setting each condition so as to satisfy the above Expression (8),even when the temperature change occurs in the photodetector 1B, theoptical fiber F10 rotates clockwise or counterclockwise in the planeorthogonal to the vertical direction around the point P while furtherkeeping the relative positions of the photodetection element 6 and theoptical fiber F10 in the horizontal direction. Thus, it is possible tofurther reduce the relative positional deviation between thephotodetection element 6 and the optical fiber F10 in the horizontaldirection caused by the temperature change. Therefore, the temperaturedependency of the detection result of the scattered light can bereduced.

In addition, without being limited to the case where both sides have thesame value as in the above Expression (8), even if the value ofα_(A)/α_(B) and the value of X_(0B)/X_(0A) are substantially the same,the temperature dependency of the detection result of the scatteredlight described above can be reduced. Substantially same refers to, forexample, the case where Expression (9) is satisfied.

X _(0B) /X _(0A)×99/100≤α_(A)/α_(B) ≤X _(0B) /X _(0A)×101/100  (9)

Expression (9) shows the case where an error between the value of as/asand the value of X_(0B)/X_(0A) is within a range of ±1%.

In addition, even in a case where the optical fiber F10 and the centerline O overlap, that is, X_(0A)=0 and X_(0B)=0, it is possible toprevent the relative positions of the optical fiber F10 and thephotodetection element 6 from changing with the temperature change asdescribed above.

As described above, according to the photodetector 1B of one or moreembodiments, the optical fiber F10 is fixed to the substrate 2 by thefirst fixing member 3 and the second fixing member 4. Further, the firstfixing member 3 is formed of a material having a positive linearexpansion coefficient, and the second fixing member 4 is formed of amaterial having a negative linear expansion coefficient. Therefore, in acase where a temperature change occurs in the first fixing member 3 andthe second fixing member 4, one of the first fixing member 3 and thesecond fixing member 4 contracts, and the other expands. Then, the firstfixing member 3 and the second fixing member 4 are disposed on bothsides of the photodetection element 6 in the longitudinal direction.Thus, in a case where a temperature change occurs in the photodetector1B, while maintaining the relative positional relationship between thephotodetection element 6 and the optical fiber F10, the optical fiberF10 rotates clockwise or counterclockwise with respect to this position.Thus, the relative positional deviation between the photodetectionelement 6 and the optical fiber F10 in the horizontal direction isreduced, and for example, it is possible to limit the relativepositional deviation between the optical fiber F10 and thephotodetection element 6 in the horizontal direction caused by expansionof both the first fixing member 3 and the second fixing member 4.Further, the contraction of both the first fixing member 3 and thesecond fixing member 4 can limit the application of tension to theoptical fiber F10.

Further, since the second fixing member 4 is formed of a material havinga negative linear expansion coefficient in the longitudinal direction,when the first fixing member 3 expands in the longitudinal direction asthe temperature rises, the second fixing member 4 contracts in thelongitudinal direction. Thus, the expansion of both the first fixingmember 3 and the second fixing member 4 in the longitudinal directioncan more reliably limit the floating of the optical fiber F10 withrespect to the substrate. Further, the first fixing member 3 contractsin the longitudinal direction as the temperature falls, and thetemperature change causes the second fixing member 4 to expand in thelongitudinal direction. Thus, the contraction of both the first fixingmember 3 and the second fixing member 4 in the longitudinal directioncan limit the application of tension to the optical fiber F10.

Further, since the absolute value of the linear expansion coefficient ofthe material forming the second fixing member 4 is larger than theabsolute value of the linear expansion coefficient of the materialforming the first fixing member 3, when the temperature of thephotodetector 1B rises, the amount of contraction of the volume of thesecond fixing member 4 exceeds the amount of expansion of the volume ofthe first fixing member 3. Mainly in the vertical direction or thehorizontal direction, this makes it possible to more reliably limit thebending of the optical fiber F10 with respect to the substrate betweenthe first fixing member 3 and the second fixing member 4. Further, thismakes it possible to more reliably limit the change in the position ofthe optical fiber F10 with respect to the photodetection element 6.

Further, in a case where the photodetector 1B is configured so as tosatisfy α_(A)/α_(B)=X_(0B)/X_(0A), even if the optical fiber F10 isshifted from the ideal design position, the optical fiber F10 is movedto rotate in a plane orthogonal to the vertical direction around a pointP in the vicinity of the photodetection element 6 with temperaturechange. Therefore, the amount of change of distance in the horizontaldirection with respect to the photodetection element 6 of the opticalfiber F10 according to temperature can be reduced. Thus, the temperaturedependency of the detection result by the photodetection element 6 canbe reduced.

It should be noted that the technical scope of the present invention isnot limited to the above-described embodiments, and variousmodifications can be made without departing from the spirit of thepresent invention.

For example, in one or more embodiments, the photodetection element 6 isfixed to the substrate 2 using the fixing base 5, but the photodetectionelement 6 may be fixed to the substrate 2 by another configurationwithout using such a fixing base 5.

Further, in one or more embodiments, the first fixing member 3 and thesecond fixing member 4 are disposed to straddle the optical fiber F10 inthe lateral direction, but the present invention is not limited to this.For example, the tip portions in the horizontal direction of the firstfixing member 3 and the second fixing member 4 may be disposed so as tobe in contact with the side surface of the optical fiber F10.

Further, the configuration described above may be applied to otherembodiments. For example, the fixing base 5 of the photodetector 1B mayhave at least one opening, and a part of the optical fiber may beaccommodated inside the fixing base 5 through the opening. Further, theopening may be closed by either the first fixing member 3 or the secondfixing member 4. Further, a part of either first fixing member 3 or thesecond fixing member 4 may be located inside the fixing base 5 (insidethe groove 5 b). Alternatively, the first fixing member 3 or the secondfixing member 4 may not be located inside the fixing base 5.Alternatively, the opening may not be closed by the first fixing member3 or the second fixing member 4.

Further, the photodetector 1B is configured such that either V₁>V₂ andV₃<V₄ or V₁<V₂ and V₃>V₄ is satisfied. Further, the photodetector 1B maysatisfy α₁/α₂=X₂/X₁.

The configuration of a photodetector 1C according to one or moreembodiments will be described below with reference to FIGS. 16 to 21.

In addition, in order to facilitate understanding of the invention, inthe drawings, the scales of components are appropriately changed.

FIG. 16 is a block diagram showing the configuration of a laser systemLS provided with the photodetector 1C of one or more embodiments. Asshown in FIG. 16, the laser system LS includes a plurality of laserdevices 31, a combiner 32 (multiplexer), an optical fiber F10 (outputoptical fiber), the photodetector 1C, and a control device 33 (controlunit). The laser system LS outputs output light L11 (laser light) fromthe output end X of the optical fiber F10.

The laser device 31 is a device that outputs a laser beam under thecontrol of the control device 33.

The combiner 32 optically combines the plurality of beams of outputlight L1 output from the plurality of laser devices 31. Inside thecombiner 32, the optical fibers F extending from respective laserdevices 31 are bundled into one (made into one by melt drawing), and theone optical fiber is fusion-spliced to one end of the optical fiber F10.The optical fiber F10 is an optical fiber functioning as a transmissionmedium, and guides the output light L11 (light obtained by opticallycombining a plurality of beams of output light L1 output from the laserdevices 31 by the combiner 32). The output light L11 guided by theoptical fiber F10 is output from the output end X of the optical fiberF10.

The control device 33 controls the plurality of laser devices 31 suchthat the power of the output light L11 output from the output end Xbecomes constant, based on the detection result to be described later ofthe photodetector 1C to be described later.

The photodetector 1C is disposed between the combiner 32 and the outputend X, and detects the power of light guided by the optical fiber F10.In addition, the photodetector 1C may be disposed between the laserdevice 31 and the combiner 32, and may detect the power of light guidedby the optical fiber F.

FIG. 17 is a perspective view of the photodetector 1C according to oneor more embodiments. As shown in FIG. 17, the photodetector 1C includesa substrate 2, a first fixing member 3, a second fixing member 4, afixing base (element fixing base) 5, and a photodetection element 6.

The photodetector 1C is located on the mounting surface 2 a of thesubstrate 2 and detects the scattered light of light guided by anoptical fiber F10 or an optical fiber F (hereinafter simply referred toas an optical fiber F10) partially placed on the mounting surface 2 a.

In addition, in one or more embodiments, a direction in which theoptical fiber F10 extends in a state before the optical fiber F10 movesdue to a temperature change is referred to as a longitudinal direction.Further, the direction perpendicular to the mounting surface 2 a of thesubstrate 2 is referred to as the vertical direction. The verticaldirection is orthogonal to the longitudinal direction. In the verticaldirection, the mounting surface 2 a side of the substrate 2 is referredto as the upper side, and the opposite side is referred to as the lowerside. Further, a direction orthogonal to the longitudinal direction andthe vertical direction is referred to as the horizontal direction.

A detailed configuration of the photodetector 1C will be described belowwith reference to FIGS. 17 to 20. FIG. 18 is a top view of thephotodetector 1C as viewed from above. FIG. 19 is a cross-sectional viewtaken along line A-A in FIG. 18. FIG. 20 is a cross-sectional view takenalong line B-B in FIG. 18.

(Fixing Base)

The fixing base 5 fixes the photodetection element 6 to the substrate 2.The fixing base 5 is configured to expand and contract at least in thevertical direction with temperature change. A vertically extendingthrough hole is formed in a central portion of the fixing base 5 in atop view (plan view). As shown in FIGS. 19 and 20, the through holesextend perpendicularly to the mounting surface 2 a of the substrate 2.The through hole has a fitting portion 5 f extending downward from theupper surface of the fixing base 5 and a reduced diameter portion 5 elocated below the fitting portion 5 f. The fitting portion 5 f and thereduced diameter portion 5 e are coaxially disposed. The inner diameterof the fitting portion 5 f is larger than the inner diameter of thereduced diameter portion 5 e. The lower end (hereinafter referred to asa positioning portion 5 d) of the fitting portion 5 f is formed in anannular shape facing upward. The positioning portion 5 d is located atthe central portion in the vertical direction of the fixing base 5. Theposition of the photodetection element 6 in the vertical direction withrespect to the mounting surface 2 a of the substrate 2 is determined bythe positioning portion 5 d.

The lower surface (hereinafter referred to as the contact surface 5 c)of the fixing base 5 is in contact with the mounting surface 2 a of thesubstrate 2. Further, as shown in FIG. 17 and the like, the fixing base5 is fixed to the substrate 2 by a screw 8. Thus, the contact surface 5c of the fixing base 5 is fixed in contact with the mounting surface 2 aof the substrate 2.

As shown in FIGS. 19 and 20, the contact surface 5 c of the fixing base5 is formed with a groove 5 b that is recessed upward. The groove 5 b isdisposed at the central portion in the horizontal direction of thefixing base 5 and partially intersects the reduced diameter portion 5 e.The groove 5 b extends over the entire length of the fixing base 5 inthe longitudinal direction. The width in the horizontal direction andthe height in the vertical direction of the groove 5 b are larger thanthe diameter of the optical fiber F10.

As shown in FIG. 20, the fixing base 5 has openings 5 b 1, 5 b 2 of thegroove 5 b. A part of the optical fiber F10 is introduced into thegroove 5 b through the openings 5 b 1, 5 b 2 and is accommodated insidethe fixing base 5. The first opening 5 b 1 is closed by the first fixingmember 3, and the second opening 5 b 2 is closed by the second fixingmember 4. A part of the first fixing member 3 enters the groove 5 bthrough the first opening 5 b 1 and is located inside the fixing base 5.A part of the second fixing member 4 enters the groove 5 b through thesecond opening 5 b 2 and is located inside the fixing base 5. A part ofthe optical fiber F10 is accommodated inside the groove 5 b. Inaddition, the groove 5 b of the fixing base 5 described above may beformed with a very small width in the longitudinal direction of theoptical fiber F10. In this case, the fixing base 5 may not have thegroove 5 b, and may be provided with only the openings 5 b 1, 5 b 2.

(Photodetection Element)

The photodetection element 6 receives the scattered light (for example,Rayleigh scattered light) from the optical fiber F10 at the bottomsurface (hereinafter referred to as the light receiving surface 6 c),and converts the intensity of the scattered light into electric power.The electric power is amplified on an electric circuit board (not shown)and input to the control device 33 (see FIG. 16). Thus, the controldevice 33 can monitor the power of the light guided by the optical fiberF10 in real time. For example, a PIN photodiode can be used as thephotodetection element 6. In a case where a PIN photodiode is used asthe photodetection element 6, the distance from the outer peripheralsurface of the optical fiber F10 to the light receiving surface 6 c isabout several millimeters.

The photodetection element 6 is formed in a cylindrical shape as shownin FIGS. 17 and 18. As shown in FIGS. 19 and 20, when the outerperipheral surface of the photodetection element 6 is fitted in thefitting portion 5 f of the fixing base 5, the positions of thephotodetection element 6 in the longitudinal direction and thehorizontal direction with respect to the fixing base 5 are determined.

In addition, when the bottom surface (hereinafter referred to as thelight receiving surface 6 c) of the photodetection element 6 abuts onthe positioning portion 5 d of the fixing base 5, the position of thephotodetection element 6 in the vertical direction with respect to thefixing base 5 is determined.

With the above configuration, the photodetection element 6 is fixed tothe fixing base 5 in a state where the positions in the longitudinaldirection, the horizontal direction, and the vertical direction withrespect to the fixing base 5 are determined. Further, since the fixingbase 5 is fixed to the substrate 2 by the screws 8, the photodetectionelement 6 is fixed to the substrate 2 through the fixing base 5. Thatis, the fixing base 5 fixes the photodetection element 6 to thesubstrate 2. In addition, since the contact surface 5 c of the fixingbase 5 is fixed in contact with the mounting surface 2 a of thesubstrate 2, the distance between the mounting surface 2 a of thesubstrate 2 and the light receiving surface 6 c of the photodetectionelement 6 in the vertical direction is determined by the length in thevertical direction from the contact surface 5 c to the positioningportion 5 d (hereinafter referred to as a light receiving surface heightH_(b)).

The first fixing member 3 and the second fixing member 4 fix the opticalfiber F10 to the substrate 2. The first fixing member 3 and the secondfixing member 4 are disposed on both sides of the photodetection element6 in the longitudinal direction. As shown in FIGS. 17 to 20, the firstfixing member 3 and the second fixing member 4 are each formed in asubstantially quarter-sphere shape. Parts of the first fixing member 3and the second fixing member 4 respectively enter the groove 5 b of thefixing base 5.

(Connection Member)

Here, as shown in FIGS. 19 and 20, the photodetector 1C of one or moreembodiments is provided with the connection member (connector) 9 fixedin contact with the mounting surface 2 a of the substrate 2. Theconnection member 9 has a placing surface 9 a on which the optical fiberF10 is placed. The connection member 9 connects the optical fiber F10located on the placing surface 9 a and the mounting surface 2 a of thesubstrate 2. The connection member 9 is disposed at a portion thatavoids the space between the mounting surface 2 a of the substrate 2 andthe contact surface 5 c of the fixing base 5. In the shown example, theconnection member 9 is disposed in a space defined by the inner wall ofthe reduced diameter portion 5 e of the fixing base 5 and the mountingsurface 2 a of the substrate 2. Thus, the placing surface 9 a of theconnection member 9 is disposed at a portion facing at least the lightreceiving surface 6 c of the photodetection element 6 with the opticalfiber F10 interposed therebetween in the vertical direction. Inaddition, for example, when the connection member 9 enters the groove 5b of the fixing base 5, the connection member 9 may not partially facethe light receiving surface 6 c.

The portion of the optical fiber F10 facing the photodetection element 6is placed on the placing surface 9 a, and the other portion of theoptical fiber F10 is placed on the mounting surface 2 a of the substrate2. In this state, by fixing the optical fiber F10 to the substrate 2 bythe first fixing member 3 and the second fixing member 4, the distance(hereinafter, simply referred to as a distance L) in the verticaldirection between the light receiving surface 6 c of the photodetectionelement 6 and the outer peripheral surface of the optical fiber F10 isdetermined.

In addition, the portions on both sides in the longitudinal direction ofthe portion of the optical fiber F10 facing the photodetection element 6are fixed to the substrate 2 by the first fixing member 3 and the secondfixing member 4. Thus, for example, the optical fiber F10 is preventedfrom floating from the placing surface 9 a and the distance L isprevented from fluctuating.

The connection member 9 may be made of, for example, a plate-like resin,and may be bonded to the mounting surface 2 a of the substrate 2 by anadhesive or the like. Alternatively, the connection member 9 may bedirectly formed on the mounting surface 2 a, by applying a UV curableresin or a thermosetting resin to be the connection member 9 on themounting surface 2 a to have a predetermined thickness (hereinafterreferred to as a connection member thickness H_(a)).

The manufacturing process of the photodetector 1C is, for example, asfollows. First, a UV curable resin or thermosetting resin to be theconnection member 9 is applied onto the mounting surface 2 a of thesubstrate 2 and cured by UV light irradiation or heating. Next, theoptical fiber F10 is placed on the placing surface 9 a of the connectionmember 9. Next, the fixing base 5 to which the photodetection element 6is attached in advance is covered from above the optical fiber F10 andthe connection member 9. At this time, a part of the optical fiber F10is accommodated in the groove 5 b of the fixing base 5. Next, the fixingbase 5 is fixed to the substrate 2 by the screws 8. Then, a UV curableresin or the like to be the first fixing member 3 and the second fixingmember 4 is applied onto the optical fiber F10 and cured, and theoptical fiber F10 is fixed to the substrate 2.

Next, the operation of the photodetector 1C configured as describedabove will be described.

As described above, the photodetection element 6 is fixed to thesubstrate 2 through the fixing base 5, and is fixed in a state where thecontact surface 5 c of the fixing base 5 is in contact with the mountingsurface 2 a of the substrate 2. Therefore, when the temperature of thephotodetector 1C rises, the fixing base 5 thermally expands, and thephotodetection element 6 moves upward with respect to the mountingsurface 2 a of the substrate 2.

On the other hand, the connection member 9 is disposed on the mountingsurface 2 a of the substrate 2, and the optical fiber F10 is placed onthe placing surface 9 a of the connection member 9. Portions of theoptical fiber F10 positioned on both sides of the connection member 9 inthe longitudinal direction are fixed on the mounting surface 2 a of thesubstrate 2 by the first fixing member 3 and the second fixing member 4.The mounting surface 2 a is located below the placing surface 9 a. Withthis configuration, the portion of the optical fiber F10 placed on theplacing surface 9 a is pressed toward the placing surface 9 a.Therefore, when the connection member 9 expands and contracts in thevertical direction, the portion of the optical fiber F10 placed on theplacing surface 9 a moves in the vertical direction in accordance withthe expansion and contraction.

Further, the placing surface 9 a is disposed at a portion facing atleast the photodetection element 6 with the optical fiber F10 interposedtherebetween in the vertical direction. Therefore, when the temperatureof the photodetector 1C rises, the connection member 9 thermallyexpands, and the portion of the optical fiber F10 facing thephotodetection element 6 moves upward with respect to the mountingsurface 2 a of the substrate 2.

As described above, in the photodetector 1C of one or more embodiments,both the fixing base 5 and the connection member 9 thermally expand asthe temperature rises, and both the photodetection element 6 and theoptical fiber F10 are moved upward to the mounting surface 2 a of thesubstrate 2.

Here, the connection member 9 is configured to expand and contract atleast in the vertical direction with the temperature change such thatthe distance L is within a predetermined range (for example, the changeamount of the distance L due to the temperature change is within ±1%).In addition, in one or more embodiments, the connection member 9 expandsand contracts at least in the vertical direction with the temperaturechange such that the distance L is maintained within the above-describedpredetermined range. Thus, as compared with, for example, the case wherethe optical fiber F10 is directly placed on the mounting surface 2 a ofthe substrate 2, it is possible to limit the relative positionaldeviation between the photodetection element 6 and the optical fiber F10in the vertical direction caused by the temperature change. Therefore,it is also possible to limit the change in the detection result of thescattered light caused by the temperature change of the fixing base 5.

In addition, with respect to a specific value within the above-describedpredetermined range, the user sets and determines a desired value, butit may be a value determined by the user in advance, specifically, aslong as the relative positional deviation between the photodetectionelement 6 and the optical fiber F10 in the vertical direction can belimited.

In addition, when the temperature of the photodetector 1C falls, boththe connection member 9 and the fixing base 5 thermally contract.Therefore, both the photodetection element 6 and the optical fiber F10are moved downward to the mounting surface 2 a of the substrate 2. Thus,as in the case where the temperature rises, it is possible to limit therelative positional deviation between the photodetection element 6 andthe optical fiber F10 in the vertical direction caused by thetemperature change.

Next, conditions for achieving the above-described effect by thephotodetector 1C of one or more embodiments more reliably will bedescribed.

First, a case where there is no connection member 9, and a portion ofthe optical fiber F10 facing the light receiving surface 6 c of thephotodetection element 6 faces the photodetection element 6 in a stateof being directly placed on the mounting surface 2 a of the substrate 2will be considered. Here, the above-described light receiving surfaceheight H_(b) at the temperature To is represented as H_(b0). Further,the linear expansion coefficient of the material forming the fixing base5 is α_(b).

When the temperature rises or fall from T₀ by ΔT, the fixing base 5thermally expands or thermally contracts, so the photodetection element6 moves in the vertical direction with respect to the mounting surface 2a of the substrate 2. The movement amount of the photodetection element6 in the vertical direction at this time is calculated by|ΔT×α_(b)×H_(b0)|. On the other hand, since the optical fiber F10 isdirectly placed on the mounting surface 2 a, the distance L fluctuatesby the amount of movement of the photodetection element 6 in thevertical direction. That is, the fluctuation amount of the distance L ina case where the connection member 9 is not provided (hereinafter simplyreferred to as “the fluctuation amount ΔL_(r) of the comparativeexample”) is expressed by the following Expression (10).

ΔL _(r) =|ΔT×α _(b) ×H _(b0)|  (10)

On the other hand, in one or more embodiments, a case where a portion ofthe optical fiber F10 facing the light receiving surface 6 c of thephotodetection element 6 is placed on the connection member 9 will beconsidered. Here, the linear expansion coefficient of the materialforming the connection member 9 is α_(a), and the thickness in thevertical direction at the temperature T₀ of the portion of theconnection member 9 on which the optical fiber F10 is placed is H_(a0).

When the temperature rises or fall from T₀ by ΔT, the fixing base 5thermally expands or thermally contracts, so the photodetection element6 moves in the vertical direction with respect to the mounting surface 2a of the substrate 2. On the other hand, the portion of the opticalfiber F10 facing the photodetection element 6 is placed on theconnection member 9, and the connection member 9 also thermally expandsor thermally contracts, so the optical fiber F10 also moves in thevertical direction with respect to the mounting surface 2 a. Themovement amount of the optical fiber F10 in the vertical direction atthis time is calculated by |ΔT×α_(a)×H_(a0)|.

From the above, in a case where the connection member 9 is provided asin one or more embodiments, the fluctuation amount of the distance L(hereinafter simply referred to as “the fluctuation amount ΔL_(e)”) is adifference between the movement amount of the photodetection element 6in the vertical direction and the movement amount of the optical fiberF10, and is expressed by the following Expression (11).

ΔL _(e) =|ΔT×α _(b) ×H _(b0) −ΔT×α _(a) ×H _(a0)|  (11)

In addition, examples of a condition for satisfying Expression (11)include that the photodetection element 6 and the optical fiber F10 movein the same direction due to temperature change, but in this respect,the linear expansion coefficient α_(b) of the fixing base 5 and thelinear expansion coefficient α_(a) of the connection member 9 may beboth positive or both negative. That is, any one of α_(b)>0 and α_(a)>0,or α_(b)<0 and α_(a)<0 may be used.

As compared with the configuration in which the connection member 9 isnot provided, the condition for limiting the fluctuation of the distanceL in the configuration in which the connection member 9 is providedaccording to one or more embodiments is that ΔL_(e)<ΔL_(r). Bysubstituting Expressions (10), (11) into both sides, the followingExpression (12) is obtained.

|ΔT×α _(b) ×H _(b0) −ΔT×α_(a) ×H _(a0)|<|ΔT×α _(b) ×H _(b0)|  (12)

The following Expression (12A) is obtained by excluding AT from bothsides of Expression (12).

|α_(b) ×H _(b0) −α _(a) ×H _(a0)|<|α_(b) ×H _(b0)|  (12A)

The following Expression (12B) is obtained by squaring the both sides ofExpression (12A).

(α_(b) ×H _(b0) −α _(a) ×H _(a0))²<(α_(b) ×H _(b0))²  (12B)

The following Expression (12C) is obtained by arranging Expression(12B).

α_(a) ×H _(a0)(α_(a) ×H _(a0)−2×α_(b) ×H _(b0))<0  (12C)

For example, in a case where the material of the connection member 9 hasa positive linear expansion coefficient (that is, in a case where α_(a)is a positive value), α_(a)×H_(a0) is a positive value, soα_(a)×H_(a0)−2×α_(b)×H_(b0) may be a negative value in order to satisfyExpression (12C). The case where α_(a)×H_(a0)−2α_(b)×H_(b0) is anegative value means a case where the amount of thermal expansion orthermal contraction of the connection member 9 does not excessivelyexceed the amount of thermal expansion or thermal contraction of thefixing base 5. That is, the provision of the connection member 9 meansthat the relative positional deviation caused by the temperature changeof the photodetection element 6 and the optical fiber F10 does notincrease.

Similarly, in a case where the material of the connection member 9 has anegative linear expansion coefficient, α_(a)×H_(a0)−2×α_(b)×H_(b0) maybe a positive value in order to satisfy the condition of Expression(12C). Similarly to the above conditions, this condition also means thatthe relative positional deviation caused by the temperature change ofthe photodetection element 6 and the optical fiber F10 does not increaseby providing the connection member 9.

From the above, since the connection member 9 and the fixing base 5 areconfigured to satisfy Expression (12C), the thermal expansion amount orthermal contraction amount of the connection member 9 excessivelyexceeds the thermal expansion amount or thermal contraction amount ofthe fixing base 5, and it is possible to limit an increase in relativepositional deviation caused by the temperature change of thephotodetection element 6 and optical fiber F10 more reliably byproviding the connection member 9. Therefore, it is possible to limitmore reliably the change in the detection result of the scattered lightcaused by the temperature change of the fixing base 5.

Next, the optimum conditions for limiting the fluctuation of thedistance L caused by the temperature change will be considered.

In one or more embodiments, the fluctuation amount ΔL_(e) is expressedby Expression (11). The condition for this to be 0 is obtained bysolving the following Expression (11A).

|ΔT×α_(b) ×H _(b0) −ΔT×α_(a) ×H _(a0)|=0  (11A)

When both sides of the above Expression (11A) are divided by AT andarranged, the following conditional expression (13) is obtained.

α_(b) ×H _(b0)=α_(a) ×H _(a0)  (13)

By designing the connection member 9 and the fixing base 5 so as tosatisfy the above Expression (13), the fluctuation of the distance L dueto the temperature change becomes zero.

Meanwhile, both sides of the above Expression (13) do not need to havecompletely the same value, and if the value of α_(b)×H_(b0) and thevalue of α_(a)×H_(a0) are substantially the same, the amount of movementof the photodetection element 6 in the vertical direction with respectto the mounting surface 2 a of the substrate 2 due to thermal expansionor thermal contraction of the fixing base 5 and the amount of movementof the optical fiber F10 in the vertical direction with respect to themounting surface 2 a of the substrate 2 due to thermal expansion orthermal contraction of the connection member 9 are substantially thesame. Thus, even if a temperature change occurs, the photodetectionelement 6 and the optical fiber F10 are displaced while maintaining therelative positional relationship in the vertical direction, and it ispossible to further limit the relative positional deviation in thevertical direction of the both caused by the temperature change.Therefore, it is possible to further limit the change in the detectionresult of the scattered light caused by the temperature change of thefixing base 5.

In addition, example of a case where the value of α_(b)×H_(b0) and thevalue of α_(a)×H_(a0) are substantially the same include a case wherethe following Expression (14) is satisfied.

α_(b) ×H _(b0)×99/100≤α_(a) ×H _(a0)≤α_(b) ×H _(b0)×101/100  (14)

The above Expression (14) shows the case where an error between thevalue of α_(b)×H_(b0) and the value of α_(a)×H_(a0) is within ±1%. Sincethe connection member 9 is configured to expand and contract in thevertical direction so as to satisfy the above Expression (14), thefluctuation amount of the distance L due to the temperature change canbe set within ±1% and can be kept.

Next, a specific example of the photodetector 1C of one or moreembodiments will be described.

In this example, the photodetector 1C is mounted so as to detect thelaser power in the optical fiber F10 of 125 μm located most downstreamof the high-power laser system LS as shown in FIG. 16. As thephotodetection element 6, a photo detector (PD) in which the currentvalue fluctuates according to the amount of received light is used. Inthis example, as a material of the fixing base 5, aluminumsurface-treated with matte black alumite is used. The linear expansioncoefficient of this aluminum is 23×10⁻⁶ [/K]. The dimensions of thefixing base 5 are 20 mm in the longitudinal direction, 20 mm in thehorizontal direction, and 60 mm in the vertical direction, and areformed in a rectangular parallelepiped shape as a whole.

A resin having a linear expansion coefficient of 69×10⁻⁶ [/K] is used asthe material of the connection member 9 of one or more embodiments. Withrespect to a plurality of photodetectors 1C in which the thickness ofthe connection member 9 in the vertical direction is changed in therange of 0 to 3 mm, a laser of constant power is guided to the opticalfiber F10, and the fluctuation of the current value of PD (hereinafterreferred to as PD current fluctuation) in a case where the temperatureof the photodetector 1C is changed from normal temperature (25° C.) to80° C. is shown in FIG. 21. The case where the thickness of theconnection member 9 in the vertical direction is 0 mm indicates the casewhere the optical fiber F10 is directly placed on the mounting surface 2a of the substrate 2 without providing the connection member 9.

The vertical axis of the graph shown in FIG. 21 is the PD currentfluctuation [%] with reference to the state at normal temperature (25°C.), and the horizontal axis is the temperature [° C.] of thephotodetector 1C.

As shown in FIG. 21, in a case where the connection member 9 is notprovided (in a case where the thickness of the connection member 9 is 0mm), when the temperature of the photodetector 1C rises to 60° C., thereis a PD current fluctuation of about −0.06%. Since the power of thelaser guided to the optical fiber F10 is constant, this means that thePD current fluctuation is caused by the fluctuation of the distance Lshown in FIG. 16, and the accuracy of the power detection result islowered according to the temperature change.

On the other hand, in a case where the connection member 9 is providedas in one or more embodiments, the value of the PD current fluctuationis limited to a small value as compared with the case where theconnection member 9 is not provided. Specifically, in a case where thethickness of the connection member 9 in the vertical direction is 1 mm,the PD current fluctuation at 60° C. is about −0.05%.

Further, in a case where the thickness of the connection member 9 in thevertical direction is 3 mm, the PD current fluctuation at 60° C. isabout +0.05%. Thus, in one or more embodiments, in order to limit the PDcurrent fluctuation at 60° C. within ±0.05%, it is found that thethickness of the connection member 9 in the vertical direction may beset within a range of 1 to 3 mm.

Further, as shown in FIG. 21, in the present example, by setting thethickness of the connection member 9 in the vertical direction to 2 mm,it is possible to substantially eliminate the PD current fluctuationaccording to the temperature. This indicates that in a case where thethickness of the connection member 9 in the vertical direction is 2 mm,both sides of the above Expression (13) have substantially the samevalue.

It should be noted that the technical scope of the present invention isnot limited to the above-described embodiments, and variousmodifications can be made without departing from the spirit of thepresent invention.

For example, in one or more embodiments, the portion of the opticalfiber F10 facing the photodetection element 6 in the vertical directionis placed on the placing surface 9 a of the connection member 9, and theother portion is placed on the mounting surface 2 a of the substrate 2,but the present invention is not limited to this. For example, theplacing surface 9 a of the connection member 9 may extend over theentire length of the substrate 2 in the longitudinal direction, and theoptical fiber F10 may be placed over the entire length of the placingsurface 9 a.

In addition, a part of the configuration described in one or moreembodiments may be applied to other embodiments. For example, in one ormore embodiments, the fixing base 5 of the photodetector 1C may have atleast one opening, and a part of the optical fiber may be accommodatedinside the fixing base 5 through the opening. Further, the opening maybe closed by either the first fixing member 3 or the second fixingmember 4. Further, a part of either the first fixing member 3 or thesecond fixing member 4 may be located inside the fixing base 5 (insidethe groove 5 b). Alternatively, the first fixing member 3 or the secondfixing member 4 may not be located inside the fixing base 5.Alternatively, the opening may not be closed by the first fixing member3 or the second fixing member 4.

Further, the photodetector 1C is configured such that either V₁>V₂ andV₃<V₄ or V₁<V₂ and V₃>V₄ is satisfied. Further, the photodetector 1C maysatisfy α₁/α₂=X₂/X₁.

Further, the configuration described above may be applied to otherembodiments. For example, in one or more embodiments, the first fixingmember 3 of the photodetector 1C may be formed of a material having apositive linear expansion coefficient, and the second fixing member 4may be formed of a material having a negative linear expansioncoefficient.

Although the disclosure has been described with respect to only alimited number of embodiments, those skilled in the art, having benefitof this disclosure, will appreciate that various other embodiments maybe devised without departing from the scope of the present invention.Accordingly, the scope of the invention should be limited only by theattached claims.

For example, in one or more embodiments, the first fixing member 3 ofthe photodetector 1A may be formed of a material having a positivelinear expansion coefficient, and the second fixing member 4 may beformed of a material having a negative linear expansion coefficient.

Alternatively, in one or more embodiments, the photodetector 1A may befixed to the substrate 2 in contact with the mounting surface 2 a of thesubstrate 2, and may include the connection member 9 having a placingsurface 9 a on which the optical fiber is placed, and connecting theoptical fiber positioned on the placing surface 9 a and the substrate 2.Further, the fixing base 5 of the photodetector 1A may have a contactsurface 5 c fixed in contact with the mounting surface 2 a, and thecontact surface 5 c may be disposed in a portion facing thephotodetection element 6 with at least the optical fiber interposedtherebetween. Further, the connection member 9 may expand and contractat least in the vertical direction with the temperature change such thatthe distance in the vertical direction between the optical fiber placedon the placing surface 9 a and the photodetection element 6 is within apredetermined range.

Similarly, a part of the contents described above may be combined withother embodiments.

REFERENCE SIGNS LIST

1A to 1C photodetector

2 substrate

2 a mounting surface

3 first fixing member

4 second fixing member

5 fixing base

5 b groove

5 c contact surface

5 d positioning portion

51 main body

6 photodetection element

9 connection member

9 a mounting surface

F optical fiber

F10 optical fiber

1. A photodetector comprising: a substrate; an optical fiber disposed onthe substrate; and a photodetection element, fixed to the substrate,that detects scattered light of light guided by the optical fiber. 2.The photodetector according to claim 1, further comprising: a firstfixing member and a second fixing member that fix the optical fiber tothe substrate, wherein the first fixing member is disposed on anopposite side of the photodetection element from the second fixingmember in a longitudinal direction of the optical fiber, the opticalfiber extends in the longitudinal direction, and either Expressions (1)and (2) are satisfied, or Expressions (3) and 4 are satisfied:V ₁ >V ₂  (1);V ₃ <V ₄  (2);V ₁ <V ₂  (3); andV ₃ >V ₄  (4), where, from a top view of the substrate, V₁ is a volumeof a portion of the first fixing member on a first side across theoptical fiber in a transverse direction orthogonal to the longitudinaldirection, V₂ is a volume of a portion of the first fixing member on asecond side opposite to the first side, V₃ is a volume of a portion ofthe second fixing member on the first side of the optical fiber in thetransverse direction, and V₄ is a volume of a portion of the secondfixing member on the second side.
 3. The photodetector according toclaim 2, wherein α₁/α₂=X₂/X₁ is satisfied, where α₁ is a linearexpansion coefficient of a material forming the first fixing member, α₂is a linear expansion coefficient of a material forming the secondfixing member, from the top view, X₁ is a distance between a center ofgravity of the first fixing member and the optical fiber in thetransverse direction, and from the top view, X₂ is a distance between acenter of gravity of the second fixing member and the optical fiber inthe transverse direction.
 4. A method for manufacturing a photodetectorincluding a substrate, an optical fiber disposed on the substrate, aphotodetection element, fixed to the substrate, that detects scatteredlight of light guided by the optical fiber, a first fixing member and asecond fixing member that fix the optical fiber to the substrate, thefirst fixing member being disposed on an opposite side of thephotodetection element from the second fixing member in a longitudinaldirection of the optical fiber, the optical fiber extends in thelongitudinal direction, the method comprising: applying a first resin,as the first fixing member, to the substrate and the optical fiber;detecting, from a top view of the substrate, a volume V₁ of a portion ofthe first fixing member on a first side across the optical fiber in atransverse direction orthogonal to the longitudinal direction, and avolume V₂ of a portion of the first fixing member on a second sideopposite to the first side; nd applying a second resin, as the secondfixing member to the substrate and the optical fiber, based on a volumedetection result of the detection of the volume V₁ and the volume V₂,wherein when applying the second resin and from the top view, adischarge amount of the second resin is controlled such that eitherExpressions (1) and (2) are satisfied, or Expressions (3) and (4) aresatisfied:V ₁ >V ₂  (1);V ₃ <V ₄  (2);V ₁ <V ₂  (3); andV ₃ >V ₄  (4), where, from the top view, V₃ is a volume of a portion ofthe second fixing member on the first side of the optical fiber in thetransverse direction, and V₄ is a volume of a portion of the secondfixing member on the second side.
 5. The photodetector according toclaim 1, further comprising: a first fixing member and a second fixingmember that fix the optical fiber to the substrate, wherein the firstfixing member is disposed on an opposite side of the photodetectionelement from the second fixing member in a longitudinal direction of theoptical fiber, the optical fiber extends in the longitudinal direction,the first fixing member comprises a material having a positive linearexpansion coefficient, and the second fixing member comprises a materialhaving a negative linear expansion coefficient.
 6. The photodetectoraccording to claim 5, wherein the second fixing member comprises amaterial having a negative linear expansion coefficient in thelongitudinal direction.
 7. The photodetector according to claim 5,wherein an absolute value of the linear expansion coefficient of thematerial of the second fixing member is larger than an absolute value ofthe linear expansion coefficient of the material of the first fixingmember.
 8. The photodetector according to claim 5, wherein a value ofα_(A)/α_(B) and a value of X_(0B)/X_(0A) are substantially the same,where α_(A) is the linear expansion coefficient of the material of thefirst fixing member, α_(B) is an absolute value of the linear expansioncoefficient of the material of the second fixing member, X_(0A) is adistance between a center of gravity of the first fixing member and theoptical fiber, and X_(0B) is a distance between a center of gravity ofthe second fixing member and the optical fiber.
 9. The photodetectoraccording to claim 1, further comprising: a first fixing member and asecond fixing member that fix the optical fiber to the substrate; and afixing base that fixes the photodetection element to the substrate,wherein the fixing base has an opening, a portion of the optical fiberis disposed inside the fixing base through the opening, and the openingis closed by either the first fixing member or the second fixing member.10. The photodetector according to claim 9, wherein a portion of eitherthe first fixing member or the second fixing member is disposed insidethe fixing base.
 11. The photodetector according to claim 9, wherein thefixing base comprises a main body that holds the photodetection element,and a width of either the first fixing member or the second fixingmember is larger than a width of the main body.
 12. The photodetectoraccording to claim 9, wherein the fixing base comprises a main body thatholds the photodetection element, and a width of either the first fixingmember or the second fixing member is smaller than a width of the mainbody.
 13. The photodetector according to claim 1, further comprising: aconnector that: is fixed to the substrate; contacts a mounting surfaceof the substrate; comprises a placing surface on which the optical fiberis placed; and connects the optical fiber disposed on the placingsurface to the substrate; and a fixing base that fixes thephotodetection element to the substrate, and that expands and contractsin a vertical direction perpendicular to a surface of the substrate inresponse to a temperature change, wherein the fixing base comprises anopening, and a contact surface fixed in contact with the mountingsurface, a portion of the optical fiber is disposed inside the fixingbase through the opening, a portion of the placing surface is disposedto face the photodetection element across the optical fiber, and theconnector expands and contracts in the vertical direction in response toa temperature change such that a distance in the vertical directionbetween the optical fiber disposed on the placing surface and thephotodetection element is within a predetermined range.
 14. Thephotodetector according to claim 13, wherein the fixing base comprises athrough hole that determines a position of the photodetection element ina vertical direction with respect to the mounting surface, andα_(a)×H_(a0)(α_(a)×H_(a0)−2×α_(b)×H_(b0))<0 is satisfied, where α_(a) isa linear expansion coefficient of a material forming the connector,α_(b) is a linear expansion coefficient of a material forming the fixingbase, H_(a0) is a thickness in the vertical direction of a portion ofthe connector on which the optical fiber is placed, and H_(b0) is alength in the vertical direction from the contact surface to thepositioning portion.
 15. The photodetector according to claim 14,wherein a value of α_(b)×H_(b0) and a value of α_(a)×H_(a0) aresubstantially the same.
 16. The photodetector according to claim 5,further comprising: a first fixing member and a second fixing memberthat fix the optical fiber to the substrate; and a fixing base thatfixes the photodetection element to the substrate, wherein the fixingbase comprises an opening, a portion of the optical fiber is disposedinside the fixing base through the opening, and the opening is closed byeither the first fixing member or the second fixing member.
 17. Thephotodetector according to claim 5, further comprising: a connectorthat: is fixed to the substrate; contacts a mounting surface of thesubstrate; comprises a placing surface on which the optical fiber isplaced; and connects the optical fiber disposed on the placing surfaceto the substrate; and a fixing base that fixes the photodetectionelement to the substrate, and that expands and contracts at least in avertical direction perpendicular to a surface of the substrate inresponse to a temperature change, wherein the fixing base comprises anopening, and a contact surface fixed in contact with the mountingsurface, a portion of the optical fiber is disposed inside the fixingbase through the opening, a portion of the placing surface is disposedto face the photodetection element across the optical fiber, and theconnector expands and contracts in the vertical direction in response toa temperature change such that a distance in the vertical directionbetween the optical fiber disposed on the placing surface and thephotodetection element is within a predetermined range.